Monday, December 22, 2014

Using the protected_content Keyword

I’ve been fiddling with some new options in Snort 2.9.7 rules. Specifically the new protected_content rule option. I discovered some things that are not clear in the Snort Manual so I thought I would share.

The protected_content option is designed to allow searching for content in a packet without having to spell out the content in the rule. That way if your rule is looking for something sensitive, you can hide this from anyone with access to the rule. It’s helpful if you’re looking for things like passwords you have used. In my case I have some content rules looking for my wife’s common passwords leaving the network. (I, of course would never re-use a password) ;-)

My old rules had the password clearly shown in the content match. So I thought this would be a perfect use case for the new keyword. However, there are some considerations. The content keyword looks through the entire packet (or whatever is entered in offset,depth,distance and within) for the string. Protected_content is different, it only looks in a specific spot. When using protected_content you search for a hash of the string instead of the string itself. So Snort has to hash whatever bytes you want to check. Because of this, we can’t really check an entire packet because it would mean calculating hundreds of hashes - way too slow.

The protected_content keyword comes with several parameters:
The hash itself
The hash type (md5, sha256, sha512)
The length of the original string
Optional - offset or distance

Consider:protected_content:"131848a7d09b05b96ea105fe238619e3"; hash:md5; length:8;
This would look in the packet at byte offset zero for an 8 byte string matching the md5 shown. It would ONLY look in the first 8 bytes. In this case the required length parameter works much like distance or within in a normal content match.

So, you need to look in a specific location. But how then do I find my wife’s password? I have no idea how far into the packet it might be.

There’s another consideration, the protected_content keyword is “computationally expensive” according to the Snort Manual. So we should precede it with a content match to take advantage of the fast pattern matcher. Turns out I can kill two birds here, I can search the entire packet and make the rule more efficient by using a content keyword prior. The answer is to search for a small subset of my protected content to determine what part of the packet to hash. Yes this does somewhat compromise my secret string but it’s a tradeoff to get detection.

I start out looking for a portion of the secret word. Hopefully this is as specific as possible without giving away too much. This is followed by the protected content option which backs up far enough to get to the start of my secret word and hash the required bytes.

Some disadvantages of this technique are:

- It’s not as fast as pure “content” but we knew that going in
- It requires that I put part of my secret word into a regular content match

Using a combination of content and protected_content I can now search for sensitive data without worrying (as much) about who has access to my rules. Of course, cracking the hash of a short string is still fairly easy but it does provide at least a bit of protection.